Aspects of a novel casting process for the production of nickel-based superalloy high pressure turbine blades

Newell, Matthew David (2009) Eng.D. thesis, University of Birmingham.

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Abstract

A novel process in which individual turbine blades are produced by the high rate solidification method has been developed. The technology, with an optimised radiation baffle, gave a thermal gradient of 11.64 x 10\(^3\) K.m\(^{-1}\), whilst maintaining a flat solidification front, calculated using a specially created and validated process model. The corresponding primary dendrite arm spacing was reduced to 165 x 10\(^{-6}\) m and the calculated freckle potential was below the critical threshold limit identified by Beckermann et al. (2000), even in highly freckle prone alloys. Low angle grain boundaries formed when misorientation accumulates in growing dendrite envelopes which subsequently converge were studied experimentally. While extensive dendrite branching or steady state growth was found to lead to an average primary dendrite misorientation of 2.3 ° that was random in nature, enhanced growth kinetics accompanying non steady state growth, found in platforms, produced a monotonic increase in accumulated misorientation of up to 10°. It was concluded that the latter is due to mechanical moments arising from extensive growth of unsupported tertiary dendrite branches growing laterally across the platform normal to the direction of gravity. The degree of misorientation is therefore dependent on local geometry and mushy zone shape.

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